Demand is increasing for phosphors used for illumination, etc. and for optical materials used for optical parts such as optical isolators. In response to an increasing demand for the use of an LED for illumination in recent years, the brightness of light-emitting diode (LED) has been increasing. A high-luminance LED emits high-intensity light and at the same time dissipates a large amount of heat as a result of feeding of a large current. Since each structural member of an LED is exposed to high-intensity light and disposed in hot places for a long period of time, it must have a high resistance to light and heat.
Typical white light-emitting devices (hereinafter referred to as white LEDs) are roughly classified into the following three types (Non-Patent Literature 1). The first type is a white LED wherein a package includes a red light-emitting diode, a green light-emitting diode, and a blue light-emitting diode. Since this type does not require a binding agent (binder) such as epoxy resin in the package, and can thus be constructed using light-emitting diodes only, it can be made to be highly resistant to light and heat. Meanwhile, it is difficult to adjust the brightness and color tone of the three light-emitting diodes, which complicates circuit configuration and thus increases manufacturing cost.
The second type is a white LED wherein a package includes: an ultraviolet (UV) light-emitting diode; and UV-excited red phosphor, UV-excited green phosphor, and UV-excited blue phosphor dispersed in a binding agent (binder), such as epoxy resin, covering the light-emitting diode. The third type is a white LED wherein a package includes: a blue light-emitting diode; and a blue light-excited red phosphor and blue light-excited green phosphor dispersed in a binding agent (binder), such as epoxy resin, covering the light-emitting diode.
The second and the third types have an advantage that the brightness and the color tone can be adjusted more easily than the first type, circuit can be simplified, and thus manufacturing cost can be reduced because they are in a structure wherein only one light-emitting diode is used in a package. Another advantage is that the color temperature adjustment range can be made large. Meanwhile, all of these types are in a structure requiring a binder. As a result of exposure of the binder to intense light and high temperature for a long time, the binder degrades and forms color, thus decreasing the light transmittance and the luminous efficiency, which are disadvantages. Furthermore, when a large current is fed to cause high-luminance emission, not only degradation of the binder but also decrease in characteristics of the phosphors may occur (Non-Patent Literature 2).
The white LEDs are not limited to those of a structure using the three luminescent colors described above. The white LED's may be constructed using two luminescent colors having a complementary color relation, namely the colors positioned opposite to each other with respect to the chromaticity coordinate of white (0.33, 0.33) of the CIE chromaticity coordinate. For example, there is a white LED wherein a package includes a blue light-emitting diode combined with particulate blue light-excited yellow phosphor dispersed in a binding agent (binder) such as epoxy resin (Patent Literature 1). Such a white LED also has a problem that its luminous efficiency decreases due to degradation of the binder.
To solve the problem of the degradation of epoxy resin, an attempt was made to use silicone resin instead, but the problem has yet to be solved completely.
Conventionally, a magnetooptic material, of the optical materials, is also called a Faraday rotator, and is used for an isolator, a circulator, etc. An optical isolator is used for the optical communication. The optical isolator has recently been used also for an optical processing apparatus. The optical processing apparatus is used increasingly to perform marking on metal, welding, and cutting, and consequently, an Yb-doped fiber laser optical processing apparatus having the oscillation wavelength of 1080 nm is becoming the mainstream. The Yb-doped fiber laser is a combination of a laser diode (LD) light source and a fiber amplifier, i.e. the low-power optical output from the LD is amplified by the fiber amplifier.
As a result, the optical isolator that can cut reflection return light having wavelength of 1080 nm efficiently, thus preventing degradation of the light source, and is highly resistant to high-power light is desired. It is necessary for such the optical isolator as followings:
(1) to have a high transmittance of light having wavelength of 1080 nm,
(2) to have a large Faraday rotation angle, and
(3) to be capable of obtaining a large single crystal.
As a material suitable to this wavelength, a terbium gallium garnet (TGG: Tb3Ga5O12) single crystal has recently been developed and put into practical use (Non-Patent Literature 3).
However, since gallium oxide, namely a raw material component of TGG, evaporates rapidly, it is difficult to increase the crystal size, improve quality, and ensure reproducibility, which was the reason why the cost cannot be decreased. As described in Non-Patent Literature 3, it is therefore desired to develop a material that has a larger Faraday rotation angle (Verdet constant) than TGG and can be produced at low cost. However, a single crystal that satisfies the above-mentioned conditions have yet to be obtained, and TGG only has been used in the market.
To solve the above problems of TGG, the development of a terbium aluminum garnet (TAG: Tb3Al5O12) single crystal was pursued. As a method of manufacturing TAG, the improved floating zone method (FZ method), which uses the laser as a heating source, is known (Non-Patent Literature 4). The TAG described in Non-Patent Literature 4 is considered superior to TGG because it has a larger Verdet constant than that of TGG. Meanwhile, since it has an inharmonious melting composition (Non-Patent Literature 3), it is difficult to grow a large crystal, which is why it has yet to be put into practical use.
After that, the research has also been conducted on the growth of a terbium scandium aluminum garnet (TSAG: Tb3Sc2Al3O12) single crystal, and there is a report that TSAG single crystal is advantageous in increasing crystal size (Patent Literature 2). The TSAG disclosed in Patent Literature 2 has a Verdet constant larger than that of TGG, meaning that a larger single crystal can be grown compared to TAG. However, compared to TGG, it is more difficult to increase the size of the single crystal.
A terbium scandium lutetium aluminum garnet (TSLAG) single crystal was then developed. The TSLAG single crystal has a Faraday rotation angle larger than that of the TGG single crystal, and crystal size was thus increased. However, since the TSLAG single crystal contains Lu, which is expensive, the high cost remains the problem to be solved. Furthermore, although its Faraday rotation angle is larger than that of the TGG single crystal, materials having yet larger Faraday rotation angle are also needed.